Metallurgical products, in particular flat products, such as sheet metal, strips or ribbons, irrespective of whether they are produced from steel, aluminium or other metals or alloys, are generally manufactured by rolling using mills formed by at least one roll stand and, for example, mills which are formed by a set of stands arranged in series to form what is known as a tandem mill train.
In general, a roll stand comprises large rotating masses, such as working rolls and backing rolls, and reducing gears. These masses may inadvertently start to vibrate, in particular when high-speed rolling is attempted.
This phenomenon, sometimes called “chatter”, which is observed more especially in cold tandem trains, resembles a resonance phenomenon because it results in vibration at a substantially fixed frequency for a given roll stand and it occurs beyond a specific speed threshold. It can cause irregularities in the thickness of the strip or breakages of the strip, or marks on the rolls. It is all the more disturbing to production since the most immediate remedy which can be applied to it is to reduce the rolling speed.
Little is known of the origin of these vibrations but it seems to reside in particular in the interactions between the traction of the strips upstream and downstream of a stand and the thickness-reduction process of the stand.
In order better to understand these phenomena, a model was made of the behaviour of roll stands and acceleration meters were placed on them. The simulations and measurements resulting from these tests showed that the most disturbing vibrations had frequencies, on the one hand, in the band from 100 Hz to 250 Hz (third octave) and in the band from 500 Hz to 700 Hz (fifth octave).
Moreover, the effects of those two types of vibration do not seem to be the same since it has been demonstrated that the vibrations of the third octave cause thickness defects and strip breakage while the vibrations of the fifth octave produce marks on the backing rolls. Furthermore, depending on the precise rolling conditions, vibration will not always start at the same frequency but in one of the indicated ranges.
In order to avoid the disadvantages of this chatter phenomenon, it is desirable to be able to detect the occurrence of these vibrations as soon as possible in order to take the necessary corrective measures, for example, reduce the rolling speed.
In order to do that, it has been proposed, for example, in BE 890928, to arrange acceleration meters on the stands, to filter the signal they emit in a suitable frequency band and to trigger a corrective action when the filtered signal exceeds a specific threshold.
Such a method enables the most important damage, such as strip breakage, to be avoided. However, the sensor, of the acceleration meter type, is very sensitive to all acceleration and the signal is generally spoilt by background noise. Thus, an endeavour was made to install it as close as possible to the site where the undesirable vibrations will occur, in general. It was also proposed to install them on the bearings of the rolling-rolls, which makes it necessary to equip all of the sets of bearings and to re-establish the connections at each change of roll.
More recently, it has been possible to install these sensors on the top of the roll stand and to record a workable signal; it is then important to attend to the processing of the signal in order to extract and detect the artefact sought.
However, this processing of the signal, in order to eliminate the background noise, generates a delay which may impair the triggering of the alarm and the corrective action at the desired moment. Furthermore, simple processing in a frequency band does not enable the frequencies of vibrations that have a harmful origin to be distinguished from those that correspond to the normal vibrations caused by some rotating masses of the installation.
In a more recent patent, EP 1 125 649, an attempt was made to remedy these disadvantages by proposing the processing of an acoustic signal coming from a microphone. The problem of the location of the sensor and its fragility is thus solved but the problem of the processing of the signal remains because a microphone detects all of the acoustic frequencies present and the signal is spoilt by major background noise. In order to eliminate the background noise, that patent proposes a processing of the signal, based on a combination of several approaches, the aim of which is to identify the occurrence of detrimental vibrations. In order to do that, it combines bandpass filters, peak detection, resonance factor calculations, Fourier analyses, and triggers an alarm when one of those parameters, or a combination thereof, exceeds a specific threshold in a predetermined frequency band.
That method has disadvantages because the time taken to process the signal is too long and it basically detects the divergence of the start of vibration of a stand of the mill. It has recently been observed that non-divergent vibrations may occur and impair the thickness or the surface state of the rolled product.
In addition, owing to the interaction of the rolling forces of each stand with the upstream and downstream tractions of each of them, the vibration phenomenon generally starts on one stand and spreads to the others. The device proposed can detect only the acoustic frequencies emitted by the stands of the tandem mill as a whole and it is not directly capable of differentiating the stands from one another.
The object of the present invention is to solve those problems by proposing a new detection method operating on the basis of a measurement signal, which method does not have the above-mentioned disadvantages and in particular does not necessitate preliminary processing in order to generate a vibration detection signal.